Interaction between the homeodomain proteins Cdx2 and HNF1 alpha mediates expression of the lactase-phlorizin hydrolase gene

Lactase-phlorizin hydrolase is a brush-border enzyme which is specifically expressed in the small intestine where it hydrolyses lactose, the main carb ohydrate found in milk. We have previously demonstrated in transgenic mice that the tissue-specific and developmental expression of lactase is control led by a 1 kb upstream region of the pig lactase gene. Two homeodomain tran scription factors, caudal-related homeodomain protein (Cdx2) and hepatic nu clear factor 1 alpha (HNF1 alpha), are known to bind to regulatory cis elem ents in the promoters for several intestine-specific genes, including lacta se, and are present in mammalian intestinal epithelia from an early stage i n development. In the present study, we examined whether Cdx2 and HNF1 alph a physically interact and co-operatively activate transcription from the la ctase-phlorizin hydrolase promoter. We show that the presence of both facto rs leads to a much higher level of transcription than the sum of the activa tion by either factor alone. The N-terminal activation domain of Cdx2 is re quired for maximal synergy with HNF1 alpha. With the use of pull-down assay s, we demonstrate a direct protein-protein interaction between Cdx2 and HNF 1 alpha. The interaction domain includes the homeodomain region of both pro teins. This is the first demonstration of a functional interaction between two transcription factors involved in the activation of a number of intesti ne-specific genes. Synergistic interaction between tissue-restricted factor s is likely to be an important mechanism for reinforcing developmental and tissue-specific gene expression within the intestine.